The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a method and apparatus for removing a substrate from the surface of a polishing pad in a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photo-resist layer placed thereon is also non-planar. A photo-resist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. Then it will be impossible to properly focus the light image onto the entire outer surface. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface for photolithography.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. CMP is a fairly complex process, and it differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing.
An effective CMP process has a high polishing rate and generates a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the CMP apparatus.
One problem encountered in the CMP process is the difficulty in removing the substrate from the polishing pad surface once polishing has been completed. As mentioned above, a thin layer of slurry is supplied to the surface of the polishing pad. When the substrate is placed in contact with the polishing pad, the surface tension of the slurry generates an adhesive force which binds the substrate to the polishing pad. The adhesive force makes it difficult to remove the substrate from the pad at the end of the polishing operation.
Typically, the carrier head is used to remove the substrate from the polishing pad. The substrate is vacuum-chucked to the underside of the carrier head. When the carrier head is retracted from the polishing pad, the substrate is lifted off of the polishing pad. However, if the surface tension holding the substrate on the polishing pad is greater than the force holding the substrate on the carrier head, then when the carrier head retracts, the substrate will remain on the polishing pad. This may cause the substrate to fracture or chip. In addition, failure to remove the substrate can cause a machine fault requiring manual intervention by the integrated circuit manufacturer. This requires shutting down the polishing apparatus, decreasing throughput. To achieve reliable operation from the polishing apparatus, the substrate removal operation should be essentially flawless.
Several techniques have been employed to reduce the surface tension between the substrate to the polishing pad. One such technique is to slide the substrate horizontally off of the polishing pad to break the surface tension before vertically retracting the carrier head. This technique may, however, damage the substrate as the substrate may detach from the carrier head as it slides off the edge of the polishing pad. The mechanical configuration of the CMP apparatus may also prohibit this technique.
Another technique is to treat the surface of the polishing pad to reduce the surface tension. However, this technique is not always successful, and the alteration of the pad surface may adversely affect the finish and flatness of the substrate, and reduce the polishing rate.
In view of the foregoing, there is a need for a chemical mechanical polishing apparatus which reliably removes the substrate from the polishing pad surface.